Uhm... to make the text readable also with a light background, is it possible to just add a dark outline around the characters? That may allow to do not need a background for the HUD at all.
For the big numbers (health and ammo), that would probably work well.
For smaller stuff, I'm not sure.
I think some HUD elements should gravitate to the edges in widescreen mode, it looks awkward to have big gaps at the edges of the screen, plus players will generally want the center of the screen (where they are looking most of the time) to be as open as possible. E.g. holdable items.
For backgrounds, I personally prefer a simple blended black (50%, but value could potentially be a cvar). Dunno about bordering, sometimes they take over and distract too much from the actual content.
Chat at bottom would be better (imo).